This invention relates to a method of welding for preventing "a crack" which is generated in an interface between an weld overlaid metal and a basic metal in a high pressure vessel. Such vessels generally contain hydrogen having high temperature and high pressure and an inner surface of the vessel is welded with a stainless steel. (The crack will be hereinafter simply referred to as an exfoliation or a disbonding.)
More specifically, this invention provides an excellent method of welding in which resistance to exfoliation, caused by high temperature and high pressure hydrogen is remarkably enhanced by selecting a particular material for a preliminary weld overlay of stainless steel.
Hydrogen having a high temperature and a high pressure is widely used in reaction vessels for ammonia production, methanol production and petrochemical refining. In order to prevent corrosion of the inner surface of such a vessel due to the substance contained therein, an austenitic or ferritic steel layer is generally weld-overlaid. (This welding process will be simply referred to as overlay-welding.) However, hydrogen may be diffused from the inner surface of the vessel to the internal portion thereof. Furthermore, super-saturated hydrogen due to a temperature drop, for example during a shut-down, is left in the vessel. Accordingly, the interface between the stainless steel and the base metal tends to be brittle to thereby generate an exfoliation or disbonding. Such a problem is a serious aspect of maintaining a pressure vessel in accordance with general safety criteria.
The factors of exfoliation are as follows:
(1) A residual strain exists because in a difference of the heat-expansion coefficients of an austenitic stainless steel and a base metal made of a ferritic steel.
(2) A hard alloy layer having a high sensitivity of hydrogen brittleness (such as martensite and low bainite) is produced.
(3) Hydrogen solubility of stainless steel is considerably different from that of the base metal. Accordingly, during a temperature drop, an internal pressure due to saturated hydrogen tends to be generated in the interface.
The above-described factors generally combine causing the exfoliation or disbonding.
Various stainless overlay welding methods were examined by the inventors. As a result, it has been recognized that a method excels where ferritic stainless steel is overlaid by welding on the basic material and thereafter at least one ordinary austenitic stainless steel is overlaid also by welding. Such a method has been disclosed in the published, unexamined Japanese Patent Application No. 52-138935.
An object of that application is to improve the above-described factors (1) and (3). Specifically, a residual strain is reduced by overlay-welding of one ferritic steel layer, on the basic material, having the same crystal composition as that of the basic material. Exfoliation is prevented by the hydrogen solubility.
Further, after the various trails and the detailed research of the exfoliation process, the inventors have found that the exfoliation tends to be generated by heat-processing after the welding. That is, if a heat-process is carried out at a temperature between 650.degree. to 700.degree. C. for an extended time after the overlay-welding, carbon contained in the basic material is diffused and transferred to the austenitic or ferritic stainless steel and then the decarbonized layer is formed in the basic material while carbon is cemented into the fused layer. The stainless steel thereby forms a hard carbonized layer. As a result, when hydrogen permeates through the stainless steel overlayer, exfoliation or disbonding is generated in the fused and carbonized layer and in the sensitive interface of the stainless steel overlaid metal thereby cooperating with the factors (1) to (3).